1. Field of the Invention
This invention relates to a reel pipe laying vessel on which a plurality of reels are disposed for laying multiple operational lines in waters having depths limited only by the strength of the pipe. More particularly, the invention pertains to a new type of vessel in which at least two reels are employed, one or more of which is used for storing and unspooling a rigid walled pipeline. The multiple reels can be used for laying a variety of lines in association with one or more rigid walled pipelines. The vessel includes a layout system which provides for simultaneous layout of multiple operational lines.
The vessel of this invention is designed to accomodate a permanently mounted pipe spooling main reel which is of substantial size and is capable of spooling pipe up to 16 inches diameter.
2. History of the Prior Art
In laying offshore subsea pipelines for such uses as the gathering of oil and/or gas from offshore wells, as, for example, in the Gulf of Mexico, it has been conventional to use one of two main methods to lay the pipe. In the first, called the "stovepiping" method, a pipeline is fabricated on the deck of a lay barge by welding together individual lengths of pipe as the pipe is paid out from the barge. Each length of pipe is about 40' or 80' long. Thus, the pay-out operation must be interrupted periodically to permit new lengths of pipe to be welded to the string. The stovepiping method requires that skilled welders and their relatively bulky equipment accompany the pipelaying barge crew during the entire layout operation; all welding must be carried out on site and often under adverse weather conditions. Further, the stovepiping method is relatively slow, with experienced crews being able to lay only one to two miles of pipe a day. This makes the entire operation subject to weather conditions which can cause substantial delays and make working conditions quite harsh.
The other principal conventional method is the reel pipelaying technique, in this method, a pipeline is wound on the hub of a reel mounted on the deck of a lay barge. Pipe is generally spooled onto the reel at a shore base. At such a shore base, short lengths of pipe can be welded under protected and controlled conditions to form a continuous pipeline which is spooled onto the reel. The lay barge is then towed to an offshore pipelaying location and the pipeline is spooled off the reel between completion points. This method has a number of advantages over the stovepiping method, among them, speed (one to two miles per hour); lower operating costs (e.g., smaller welding crews and less welding equipment must be carried on the lay barge); and less weather dependency.
The broad concept of reel pipelaying was also disclosed in British Pat. No. 601,103 (Ellis), issued Apr. 28, 1948, wherein it was suggested that lengths of pipe be joined together at the manufacturing plant and coiled onto a drum, mounted on a barge or ship; the loaded barge would then be moved to the desired marine location and the pipe unwound from the drum by fixing one end of the pipe and towing the barge away from the fixed location.
After a hiatus of about thirteen years, research into the reel pipelaying technique was renewed and was carried on by Gurlter, Hebert & Co., Inc. of New Orleans, La. By 1961, that company had sufficiently advanced the reel pipelaying technique to make it a commercially acceptable and viable method laying pipe in the offshore petroleum industry, able to compete with the traditional stovepiping technique. The first known commercial pipelaying reel barge, called the U-303, was built by Aquatic Contractors and Engineers, Inc., a subsidiary of Gurtler, Hebert, in 1961. The U-303 utilized a large vertical axis reel, permanently mounted on a barge and having horizontally oriented flanges (generally referred to in the trade as a "horizontal reel"). A combined straightener-level winder was employed for spooling pipe onto the reel and for straightening pipe as it was unspooled. The U-303 first laid pipe commercially in September 1961, in the Gulf of Mexico off the coast of Louisiana and was used successfully during the 1960's to lay several million linear feet of pipe of up to 6" diameter. The U-303 reel pipelaying barge is described in U.S. Pat. No. 3,237,438 (Tesson) and U.S. Pat. No. 3,372,461 (Tesson) both assigned to the assignee of the invention hereof.
The successor to the U-303, currently in use in the Gulf of Mexico and known in the trade as the "Chickasaw" also utilizes a large horizontal reel, permanently mounted to the barge such that it is not readily movable from one carrier vessel to another. Various aspects of "Chickasaw" are described in the following U.S. Patents, all assigned to the assignee of the invention hereof:
Sugasti, et al.--U.S. Pat. No. 3,630,461
Gibson--U.S. Pat. No. 3,651,778
Mott, et al.--U.S. Pat. No. 3,680,342
Key, et al.--U.S. Pat. No. 3,712,100
The Gibson patent shows an apparatus for diverting a single pipeline from a horizontal unspooling direction to a vertical direction for layout in a body of water. While the patent mentions that more than one reel could be employed there is no enabling disclosure from which a multiple reel vessel could be constructed and operated.
U.S. Pat. No. 3,685,306 to Mott also describes an apparatus which diverts a single pipeline from a horizontal position to a vertical direction. The pipeline can be successively unreeled from adjacent ganged reels.
Commercial reel pipelaying techniques require the use of certain pipe handling equipment in addition to the reel. Among such pipe handling equipment usually employed in commercial reel pipelaying systems is a straightener mechanism. This may take the form of a series of rollers or tracks, or any other arrangement which imparts sufficient reverse bending force to the pipe to remove residual curvature so that after unspooling, the pipe will lay substantially straight on the sea bottom.
U.S. Pat. No. Re. 30,846 (Lang et al) describes an apparatus for laying pipe from a vertical reel in which the pipe conditioning apparatus is pivotable to adjust the lift-off angle of the pipe relative to the horizontal (e.g., the deck of a ship) as a function of the water depth in which the pipe is being laid. This has distinct commercial advantages, especially where the reel pipelaying system is incorporated into a self-propelled ship, such as that of the present invention, capable of traveling to different job sites, having different pipe size and/or lay depth requirements.
An early concept for a reel pipelaying ship is described in Goren, et al. "The Reel Pipelay Ship--A New Concept" Offshore Technology Conference Proceedings, May 1975 (Paper No. - OTC 2400). This paper (hereafter the Goren, et al 1975 OTC Paper) describes advantages and operating features of a proposed reel pipelaying ship. However, the cost of construction of a ship as described there was estimated to be on the order of $1,000,000,000.